Brushes Used in DC Generator

Q.Why is carbon used in the brushes in a DC generator?

The inductances of the energized windings, and the mechanics of a brush type motor or generator make arcing and sparking an unavoidable characteristic of the machine at the brush interface. Electrical arcs produce very high temperatures. High enough to melt metals. 

Voltage drop across brushes is 1-2 volts

 

Carbon has a very high melting point (~3500degC) compared with other conductors. Brushes made with copper or steel would wear out faster because of factors including:

1) The melted metal will break away from, or even be vaporized, leaving the brush with less material to do its job.
2) If the melted material doesn't vaporize or otherwise move away from the interface, it creates a mess for the brush. It can weld together bristles of the brush. It can build up material that interferes with the motion of the rotor.
3) As the metal melts in the presence of an arc, conductive material can be liberated, therby prolonging the arc, exasperating the wearing action.

Carbon is less prone to those high temperature effects and associated impacts to the generator operation.

Also, Graphite is conductive and auto-lubricant (low friction coefficient) so, it provides a good dynamic contact to rings with minimum erosion. 


Carbon Graphite - Carbon Graphite brushes have been used in motor applications since the use of actual copper wire brushes that wore away the commutator. This material is mainly used as a base for impregnating metal graphite brushes today. Carbon graphite can be used in lower current density and lower speed applications because of its high resistivity.

Electrographitic – Electrographitic brushes are baked in excess of 2000°C to increase the strength of the material. These can stand up to very high current densities and high speed applications. With a lower resistance, electrographitic brush grades can also tolerate higher temperatures. There are many different grades in this material and it is the most widely used brush on the market today.

Graphite – Graphite brushes are available in several grades with different resistances and strengths needed for specific applications, such as high speed turbo generators and slip ring motors. Generally low to medium current density and lower speed applications use this grade as a cheaper alternative to electrographitic or metal graphite brushes.

Metal Graphite – Metal graphite brushes offer very low resistance and high current density which means a much lower voltage drop across the brush. These are used commonly in low voltage machines that require these specifications such as DC motors, low voltage generators, and slip ring inductions motors. The low resistance also offers less EMI (electromagnetic interference) if that may be a problem for a particular industrial application around sensitive equipment.